1. Field of the Invention
This invention relates to and has among its objects the provision of novel methods for fractionating polysaccharidic substances into components of differing activity. It is a particular object of the invention to prepare high activity heparin in large quantities using a novel lectin-containing, water-insoluble gel matrix to which antithrombin and heparin are applied. Further objects of the invention will be evident from the following description wherein parts and percentages are by weight unless specified otherwise.
2. Description of the Prior Art
Heparin is a glycosaminoglycan, having uronic acid, glucosamine, and sulfate moieties, that functions as a blood anticoagulant by binding to the inhibitor antithrombin and accelerating the rate at which this inhibitor neutralizes serine proteases of the coagulation mechanism.
The injection of unfractionated heparin alone (either intravenously or subcutaneously) is routinely employed for the treatment of thromboembolism or for the prevention of clot formation in at-risk patients. Despite its widespread and growing use for these purposes, problems concerning side effects and efficacy have been pointed out. Some of the problems associated with current anticoagulant therapy are the following:
(1) Patients often demonstrate widely different dose responses to administered heparin. This requires a rather individualized treatment procedure and constant monitoring of the resulting clotting characteristics. Quite often, the desired dosage is exceeded which necessitates the neutralization of the excess heparin.
(2) Heparin, as it is used clinically, is approximately 30% active as an anticoagulant (as defined by its ability to interact with antithrombin which results in the rapid inhibition of thrombin and other coagulation proteases). The majority of administered heparin (70%) shows no particular affinity for antithrombin but can interact with many other plasma proteins with consequences that may be undesirable. The best example of this phenomenon is the activation of lipoprotein lipase which results in the clearance of circulating triglyceride.
(3) Heparin is a highly charged polyanion and is capable of many non-specific electrostatic interactions with plasma proteins, blood cells, and endothelial surfaces. Upon injection heparin becomes distributed among these components. Although antithrombin binds in a specific fashion and with high affinity to the active fraction of the total heparin, is is unlikely that all of the anticoagulant heparin binds to the plasma antithrombin. Thus, the actual anticoagulant dosage of heparin received during heparin therapy is a complex function of any number of equilibria which reduce the amount of productive heparin-antithrombin complexes formed.
(4) Heparin has been implicated as a cause of thrombocytopenia due to its interaction with platelets in patients undergoing prolonged anticoagulant therapy.
(5) Circulating antithrombin levels have been shown to decrease as a result of prolonged administration of heparin. Antithrombin levels lowered in this way are reported to remain depressed for several days following the end of treatment. This may be a particularly undesirable effect in patients predisposed to thrombosis.
(6) In patients with congenital antithrombin deficiency, the administration of heparin may not be completely efficacious.
The administration of antithrombin has also been proposed to be a means of controlling undesirable clot-formation in at-risk patients. Those who might benefit most from this therapy would be those congenitally deficient in antithrombin as well as individuals undergoing certain types of surgery. In order for this type of therapy to be effective, however, very large amounts of antithrombin would be required. Also, treatment of congenital antithrombin deficients with antithrombin concentrates would require large amounts of this protein at frequent dosages since the plasma half-life of antithrombin is about three days.
Fractionation of heparin into high and low activity components is difficult because heparin species possessing active chain sequences are virtually indistinguishable from those possessing inactive chains. However, heparin has been separated into high activity and low activity components by sucrose density gradient centrifugation of heparin mixed with antithrombin-heparin cofactor (Lam et al, Biochemical and Biophysical Research Communications, 1976, Vol. 69, No. 2, pages 570-577). Heparin also has been fractionated by affinity chromatography on immobilized antithrombin (Hook et al, FEBS Letters, 1976, Vol. 66, pages 90-93. In this method antithrombin is coupled covalently with a cyanogen bromide-activated, water-insoluble matrix, such as, for example, Sephadex.RTM., Sepharose.RTM., etc. Heparin is applied to the immobilized antithrombin material, which adsorbs the high-activity heparin species. After separation of the matrix containing the adsorbed high activity component from the low activity heparin component, the matrix is treated with a high salt medium to elute the high activity heparin species therefrom.
An alternative method involves the separation of heparin-antithrombin complexes from unbound heparin by gel chromatography on Sephadex.RTM. G100. However, due to the size heterogeneity inherent in commercial heparin preparations and the resulting broad chromatographic profile of the heparin itself, the above method must employ heparin fractions of defined molecular weight in order to permit the separation of the heparin-antithrombin complex. This has been accomplished with a low molecular weight heparin species having an average molecular weight of 6000 daltons (Rosenberg et al, Proc. Nat. Acad. Sci., 1978, Vol. 75, No. 7, pages 3065-3069). In this case, a heparin-antithrombin complex was separated from free heparin in an initial gel chromatographic step and was subjected to a second chromatography in the presence of high salt to disrupt the complex. The high affinity heparin obtained in this sequence had a specific anticoagulant activity of about 360 units/mg compared to the starting pool of 96 units/mg. A low affinity heparin pool of 4 units/mg was also obtained by repetitive depletion of the starting material.
A complex of antithrombin and high molecular weight, high affinity heparin was also prepared by gel chromatography on Sephadex.RTM. G100 (Rosenberg et al, B.B.R.C., 1979, Vol. 86, No. 4, pages 1319-1324). In this instance, the complex was separated from excess antithrombin for the purpose of analytical characterization of the ratios contained and required the use of a heparin species previously fractionated both for size and activity.
One major problem confronting workers in all of the above-described methods is that the fractionation or preparation has been accomplished only on a laboratory scale. In other words, large scale manufacture or manufacture of pharmaceutically useful amounts has not been realized either because of limitations inherent in the method or because of the limited availability of antithrombin.
Fractionation of heparin into high and low activity components is complicated further by the fact that antithrombin coupled to a water-insoluble matrix cannot be recovered without substantial or total destruction of the antithrombin. This results because the antithrombin is covalently bound to the matrix by means of, for example, a cyanogen bromide coupling process, and the conditions necessary to cleave the coupling destroy the antithrombin.
Recently, a new method for the measurement of the binding of ligands to solubilized membrane receptors, such as a receptor for epidermal growth factor-urogastrone (EGF-URO) was described by Nexo et al, J. Biol. Chem., 1979, Vol. 254, No. 18, pages 8740-8743. The soluble receptor is first immobilized on lectin-agarose beads and ligand binding is then determined on the bead-bound receptor. The chromatographic and binding properties of solubilized receptor can be studied due to the restoration of the ligand recognition property of the receptor. After the solubilized receptor is immobilized on lectin agarose, the binding of a ligand, such as EGF-URO, to the immobilized receptor is rapid, reversible, peptide specific, and of high affinity. The author notes that his method deserves consideration for the study of any receptor that recognizes a ligand free of carbohydrate.